Answer:
They attacked the Allied troops at Bulge because they wanted to make another great offensive which would allow them to sign a peace treaty under good conditions.
Step-by-step explanation:
After Operation Overlord Allied troops deliberated France and started moving towards Benelux. German leaders realized that they need to react fast if they want to prevent capitulation. Therefore almost half of million soldiers were involved in the operation to attack Allied troops at Bulge and to try to split them. Although, at the beginning they were successful, they failed in their task after a month of fighting.